A pervasive characteristic of mental retardation is response slowness and variability. Much research utilizing the reaction time (RT) task has implicated attending/arousal functions and the response performance as sources of this characteristic. The proposed research will attempt to attack this problem by measuring the latencies of specific physiological components (evoked potentials, muscle potentials) in the stimulus-response chain. Each of these components, and the RT, will be manipulated by procedures designed to affect each one differentially. This will isolate those components in the slow retardate and normal which contribute to slowness and variability. In addition, cortical motor potentials will also be explored for clues to RT slowness. Anayses will entail an intensive within-subject comparison of response speed extremes. This information will be obtained on mental retardates and three age samples of normals - the latter to obtain a developmental perspective on the brain-behavior relations involved.